In late years, there are eagerly developed electronic devices (compound semiconductor devices) in which a GaN layer and an AlGaN layer are formed sequentially on a substrate made of sapphire, SiC, GaN, Si, or the like, and the GaN layer is used as a channel layer. GaN has a band gap of 3.4 eV, which is larger compared to 1.4 eV of GaAs. Accordingly, the compound semiconductor device is expected to operate with a high breakdown voltage.
Amplifiers for base station of mobile phones are required to operate at high voltage for improving current efficiency, and thus improvement in breakdown voltage is needed. Currently, there has been reported a value higher than 300 V as a breakdown voltage when current is off in a GaN-based high electron mobility transistor (HEMT), which is used in the amplifiers for base station. Further, there has been reported a value higher than 200 V as a breakdown voltage when current is off also in an HEMT used in an extremely high frequency band.
FIG. 1 is a cross-sectional view illustrating a structure of a conventional GaN-based HEMT. On an SiC substrate 101, there are sequentially formed an AlN layer 102, a non-doped i-GaN layer 105, a non-doped i-AlGaN layer 106, an n-type n-AlGaN layer 107, and an n-type n-GaN layer 108. Moreover, an SiN layer 109 is formed on the n-GaN layer 108. An opening is formed in the SiN layer 109, and a gate electrode 111g is formed therein. In the n-GaN layer 108 and the SiN layer 109, two openings are further formed with the gate electrode 111g being interposed therebetween. A source electrode 111s is formed in one of the openings, and a drain electrode 111d is formed in the other. Incidentally, the AlN layer 102 functions as a buffer layer. The gate electrode 111g is in Schottky contact with the n-GaN layer 108, and the source electrode 111s and the drain electrode 111d are in ohmic contact with the n-AlGaN layer 107.
When such a conventional GaN-based HEMT is used as an electronic device with a high breakdown voltage, its characteristics may vary largely. For example, when turning on/off a high frequency power operation is repeated, the output thereof may drift. This phenomenon will be described.
FIG. 2 is a circuit diagram illustrating a structure of a circuit having the GaN-based HEMT. In the circuit, a source of the transistor (GaN-based HEMT) Tr is grounded, and to a drain thereof, one end of an inductor L and one end of a resistor R are connected. A direct current (DC) bias Vd is applied to the other end of the inductor L. Further, the other end of the resistor R is grounded. To a gate of the transistor Tr, an alternating current (AC) power supply P is connected, which applies an AC signal RF of −2 V to 4 V. Incidentally, to the gate of the transistor Tr, a gate voltage Vg of −1 V is applied during an off time, in which the AC signal RF is not applied.
When such a circuit is used for an amplifier for base station, the DC bias Vd is set to about 50 V, and the average value of drain current is set to about 2% to 3% of a maximum value. When a high-frequency signal (AC signal RF) of about 2 GHz is applied to the gate of the transistor Tr, a current-voltage characteristic as illustrated in FIG. 3 is obtained. The horizontal axis in FIG. 3 is a drain (drain-source) voltage, and the vertical axis is a drain (drain-source) current.
Further, in an amplifier for base station, on/off of the transistor Tr is switched frequently. For example, control as illustrated in FIG. 4A is performed. The vertical axis in FIG. 4A is a value of DC drain current at a bias point. The quiescent current value of 10 mA/mm is a current value set in advance, which flows when the high-frequency signal is off, and the average value of 150 mA/mm is the average value of the drain current when the high-frequency signal is on.
However, when it is attempted to perform control as illustrated in FIG. 4A, the current drops excessively in practice when the high-frequency signal is turned off, as illustrated in FIG. 4B, and a sufficient output (current of 150 mA/mm) cannot be obtained when the high-frequency signal is turned on thereafter. That is, a drift phenomenon of output occurs. Due to such an excessive drop, the current may become about 1 mA/mm to 2 mA/mm. The drop of current recovers over time, but a long time period of one or more minutes is needed for recovering to a degree that the output becomes stable. Therefore, a time period of one or more minutes is needed for the DC bias to recover to the original state, which may hinder intermittent on/off operation of the high-frequency signal. Such an excessive response characteristic exists in the conventional GaN-based HEMT illustrated in FIG. 1.
A technique to suppress the drift phenomenon of output accompanying such an excessive response characteristic is discussed in Japanese Laid-open Patent Publication No. 2006-147663. FIG. 5 is a cross-sectional view illustrating a structure of a conventional GaN-based HEMT discussed in Japanese Laid-open Patent Publication No. 2006-147663.
In the GaN-based HEMT, an AlGaN layer 103 is provided between the AlN layer 102 and the i-GaN layer 105 of the GaN-based HEMT illustrated in FIG. 1.
In such a GaN-based HEMT illustrated in FIG. 5, crystallinity of the i-GaN layer 105 improves as compared to the GaN-based HEMT illustrated in FIG. 1. Accordingly, two-dimensional electron gas captured in a trap existing in a lower portion of the i-GaN layer 105 can be emitted easily, and the drift phenomenon of output accompanying the excessive response characteristic is suppressed. FIG. 6 is a graph illustrating drift phenomena of output of the GaN-based HEMT illustrated in FIG. 1 and the GaN-based HEMT illustrated in FIG. 5. The solid line in FIG. 6 depicts the characteristic of the GaN-based HEMT illustrated in FIG. 1, and the chain and dot line depicts the characteristic of the GaN-based HEMT illustrated in FIG. 5. When the quiescent current value of the drain current during an off-time of application of the high-frequency signal is 10 mA/mm, a recovery of the drain current to about 9 mA/mm allows to obtain sufficient output upon application of the next high-frequency signal. Then as illustrated in FIG. 6, even if the drain current decreases to about 2 mA/mm during an off time of application of the high-frequency signal, the drain current recovers to about 9 mA/mm by about four seconds.
Further, a technique to suppress the drift phenomenon of output is discussed also in Japanese Laid-open Patent Publication No. 2008-251966. In the technique, the surface of the AlN layer corresponding to the AlN layer 102 of the GaN-based HEMT illustrated in FIG. 1 is made coarse.
By the techniques discussed in Japanese Laid-open Patent Publication No. 2006-147663 and Japanese Laid-open Patent Publication No. 2008-251966, the initial objects can be achieved. However, for realizing quicker operations, it is necessary to recover the dropped drain current more quickly.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-114653    Patent Document 2: Japanese Laid-open Patent Publication No. 2006-147663    Patent Document 3: Japanese Laid-open Patent Publication No. 2008-251966    Patent Document 4: Japanese Laid-open Patent Publication No. 2008-205146